Apparatus for preparing pure carbon dioxide (co2) from a gaseous carbon dioxide containing product

ABSTRACT

The invention relates to an apparatus for preparing pure carbon dioxide (CO 2 ) from a gaseous CO 2  containing product, which apparatus comprises a processing line which comprises at least an inlet for the gaseous product, means for removing water-soluble impurities from the product, compression means for compressing and pressurising the product, cleaning means for removing further impurities from the product, as well as storage means for collecting and storing the purified CO 2 , which elements are interconnected by means of suitable piping. According to the invention, the compression means comprise at least one screw compressor so as to provide an apparatus for preparing pure carbon dioxide from a gaseous CO 2  containing product which has a higher efficiency, which is of simpler, cheaper construction and which is easy to maintain.

DESCRIPTION

[0001] The invention relates to an apparatus for preparing pure carbondioxide (CO₂) from a gaseous CO₂ containing product, which apparatuscomprises a processing line comprising at least an inlet for the gaseousproduct, means for removing water-soluble impurities from the product,compression means for compressing and pressurising the product, cleaningmeans for removing further impurities from the product, as well asstorage means for collecting and storing the purified CO₂, whichelements are interconnected by means of suitable piping.

[0002] Such an apparatus is known, for example from European patent no.EP-B-0 646 756 granted to the present applicant. During the preparationprocess of beverages, in which fermentation processes take place, suchas the brewing of beer and the like, a considerable amount of gas isreleased in the form of carbon dioxide (CO₂).

[0003] Because in particular a beer brewer needs CO₂ for filling thebrewed beer into containers, rinsing out piping, etc., and because inaddition to that the gas that has been released during the brewingprocess is currently being used, after being collected and purified, forproducing carbonated (soft) drinks, the gas is not blown off butcollected and purified.

[0004] To this end, an apparatus or a CO₂ recovery plant in accordancewith the introduction and as disclosed in, for example, EP-B-0 646 756is used. The CO₂-containing gas is caught by means of said known plantand all kinds of components, such as the organic and sulphurouscomponents produced during the fermentation process, air and foam,water, etc. are removed therefrom in a number of processing steps untilcarbon dioxide having a high degree of purity is obtained.

[0005] Another application of such an apparatus is the winning of pureCO₂ from, for example, process gas from the chemical industry, naturalsources, etc.

[0006] The main component of the CO₂ recovery apparatus is formed by thecompression means, which currently consist of non-lubricated pistoncompressors. The drawbacks of the use of piston compressors in thecurrent apparatus include in particular their overall dimensions, aswell as their weight in relation to the other components of the recoveryplant. Since it is not possible to use oil-lubricated piston compressorsfor preparing pure “food-grade” carbon dioxide, non-lubricated pistoncompressors are used. Partly because of the multitude of moving parts itcomprises, this type of piston compressor requires a great deal ofmaintenance, whilst it is furthermore characterized by a higher failurerate.

[0007] The object of the invention is to provide an apparatus for thepreparation of pure carbon dioxide from a gaseous CO₂ containing productwhich does not exhibit the aforesaid drawbacks. In order to achieve thatobjective, the apparatus according to the invention is characterized inthat the compression means comprise at least one screw compressor. Theuse of a screw compressor instead of a piston compressor in a CO₂recovery plant has a number of advantages, in particular its greaterefficiency compared with the piston compressors that are currently beingused. In addition to that, the screw compressor is characterized by itscompact construction, both as regards its weight and as regards itsdimensions, whilst it comprises fewer moving parts, which reduces therisk of malfunction and failure.

[0008] A special embodiment of the apparatus according to the presentinvention is characterized in that the screw compressor is awater-cooled screw compressor. As a result, the moving parts are notonly cooled but also “lubricated” by a medium which is not harmful tothe final product and to the consumer in such a “food-grade”application.

[0009] According to the invention, separating means may be arrangeddirectly after the screw compressor for separating the water from thecompressed product, wherein the water outlet of the separating means maybe connected to the water inlet of the water-cooled screw compressor.This embodiment has the additional advantage that the means for removingwater-insoluble impurities from the product can thus form part of thewater separating means of the screw compressor. As a result, alower-cost apparatus of even more compact construction is obtained,since two essential parts of the apparatus can now be integrated witheach other.

[0010] In a specific embodiment of the apparatus according to theinvention, the separating means comprise a cyclone.

[0011] In order to realise a more effective manner of pressurising thegaseous CO₂-containing product, the compression means comprise two screwcompressors which are arranged in series, wherein one driving means maybe used for driving the two screw compressors. Said driving means maydrive the screw compressors by means of one belt transmission or bymeans of separate belt transmissions.

[0012] By gearing the two screw compressors to each other via thedriving means during operation, a two-stage compression stroke can beobtained, by means of which the CO₂-containing gaseous product can becompressed to a high pressure, which is highly desirable in connectionwith the purification steps further down the apparatus. In particularthe efficiency of the apparatus and as well as the yield of pure CO₂ canthus be enhanced significantly.

[0013] The invention will now be explained in more detail with referenceto a drawing, in which:

[0014]FIG. 1 shows an overall diagram of an apparatus according to theprior art;

[0015]FIG. 2 shows one aspect of the apparatus according to theinvention;

[0016]FIG. 3 is an exploded view of a screw compressor for use in anapparatus according to the invention;

[0017]FIGS. 4a-4 c are further views of a screw compressor for use in anapparatus according to the invention.

[0018] For the sake of clarity, like parts are indicated by the samenumerals in the various figures.

[0019]FIG. 1 schematically shows an apparatus for recovering CO₂ from agaseous CO₂-containing product. Said gaseous CO₂-containing product isgenerally a gas or gases released in the course of a beer-brewingprocess. Said gas contains all kinds of components which must be removedin order to obtain “food-grade” carbon dioxide as the final product.Said “food-grade” carbon dioxide can be used for filling bottles withbeer or for producing and bottling (soft) drinks. The reason why thepurified carbon dioxide must have the “food-grade” classification isthat, in addition to CO₂, also all kinds of other fermentationcomponents, such as various alcohols as well as organic and sulphurouscomponents are produced during the process of brewing beer. Saidcomponents and aromas influence the taste and the smell of the finalproduct which are treated with the CO₂. Consequently, it is desirable toobtain CO₂ having the highest degree of purity.

[0020] The gas that is obtained during the brewing process is passedthrough a so-called foam trap via the pipe 10 a. The foam that haslikewise been produced during the fermentation process must first beremoved from the gas, because the foam, which mainly consists of air,may damage certain mechanical, usually moving parts (such as valves, andalso the screw compressor) further down the apparatus. The gas is ledinto the foam trap 10 at the bottom via the pipe 10 a, after which wateror atomized moisture is sprayed into the foam trap via the pipe 10 c.This makes it possible to discharge the foam, whilst defoamed gas can bestored in the storage tank 11 via the pipe 10 b, which storage tankfunctions as a buffer for creating a stable volume flow of carbondioxide through the apparatus.

[0021] The defoamed gas is introduced into a so-called gas washing orrinsing device 17 via the pipe 12 a, wherein the alcohols and otherwater-soluble components are rinsed or washed out of the CO₂ flow bymeans of water according to the so-called counterflow principle, whichwater from the inlet A is sprayed into the gas washing installation 12from above by the spray nozzles 12 c. The rinsing water and the alcoholsand other soluble components dissolved therein are collected at thebottom of the gas washing installation 12 and discharged via the pipe B.The “washed” CO₂ flow leaves the gas washing installation 12 via thepipe 12b and is carried to the compression means 13 which, in accordancewith the prior art, is in the form of a non-lubricated pistoncompressor.

[0022] Since the working pressure of the apparatus is usually 18 bar,the piston compressor operates in two stages, wherein the CO₂ iscompressed to about 3.5 bar in the first stage and to the requiredprocess pressure of 18 bar in the second stage. Preferably, compressiontakes place in two stages so as to prevent the temperature of the gas inthe compressor 13 running up too high. If the gas temperature is toohigh, damage to the moving parts may occur. To this end, the gas iscompressed to about 3.5 bar in the first compression stage andintroduced into an intercooler 14 a via the pipe 13 a. After cooling hastaken place, the CO₂ can be carried to the second stage via the pipe 14a′, in which it is further compressed to the required process pressureof 18 bar.

[0023] The water (from the gas washing installation 12) that may bepresent in the gas can be separated in the form of condensate at thefirst compression stage via the discharge pipe 14 c. Also after thesecond compression stage, the CO₂ is introduced into a second condenser14 b via the pipe 13 b, in which condenser further cooling of the gastakes place and condensate that has remained behind can be dischargedvia the discharge pipe 14 c′. The cooling medium that is required forthe intercoolers or condensers 14 a and 14 b is supplied via the coolingpipe A, A′ and discharged via the return pipes B.

[0024] After the compression stroke, the CO₂ flow is passed throughfirst filter means, preferably an active carbon filter 15 a, under aprocess pressure of 18 bar. The active carbon in said filter means 15 aabsorbs the impurities from the CO₂ flow that are not soluble in water.Said impurities may be all kinds of gaseous sulphur components, forexample. After the active carbon filter 15 a, the purified CO₂ flow ispassed through second filter means 15 b in order for the moisture thatmay still be present in the CO₂ to be removed. This is necessary becausethe dew point of the CO₂ flow is still too high to enable condensationof moisture that is still present. By passing the CO₂ flow through afilter 15 b containing a drying agent, the dew point of the CO₂ flow islowered to a value below −64° C., so that the moisture which is presentwill condense and be absorbed in the filter.

[0025] It will be understood that the two filter means 15a and 15b willbecome saturated in the course of time, so that the flow must bediverted to new filters 15 a and 15 b, respectively, is necessary. Tothat end, a second line comprising series-arranged filter means 15 a′and 15 b′ (not shown) is incorporated in the apparatus parallel to theseries-arranged filter means 15 a and 15 b, to which the CO₂ flow can beled the moment the first filter line (15 a and 15 b) has becomesaturated and must be shut down in order to be cleaned. The switch fromthe first line 15 a and 15 b to the second filter line 15 a′ and 15 b′(not shown) takes place every 12 hours, for example, so that the CO₂recovery plant can be operated continuously.

[0026] In a particular application of the recovery plant, thesubstantially pure CO₂ flow is then led to a condenser 16 via the pipe15, in which condenser the CO₂ flow is cooled down to a temperaturebelow −24° C. by means of a cooling coil, through which a cooling medium(not shown) flows. The fact is that said temperature constitutes the dewpoint of carbon dioxide with a process pressure of 17.5 bar. The CO₂that condenses in the condenser will collect at the bottom of thecondenser 16 and flow to the storage tank 17 via the intermediate pipe16 a under the influence of the force of gravity.

[0027] Although this is not shown in FIG. 1, the storage tank 17 mayfurthermore comprise means which heat the liquid CO₂, as a result ofwhich the CO₂ is purified in the intermediate pipe 16 a, in particularif said intermediate pipe is provided with a gasket. The impure gaseousCO₂ may be used as a so-called “purging” gas for regenerating orcleaning the active carbon filters and the drying filters (15 a, 15 band 15 a′, 15 b′ respectively) in the first and the second filter line.The degree of purity of the liquid CO₂ is now very high, and afterstorage in the tank 17 the CO₂ is carried off, via pipe 17 a, forfurther processing, for example for filling beer bottles or for bottlingcarbonated (soft) drinks. This very pure CO₂ has the “food-grade”classification.

[0028] One of the drawbacks of the use of a piston compressor 13 in theprior art CO₂ recovery plant is the fact that it takes up a considerableamount of space. In addition to that, a non-lubricated piston compressormust be used at all times, because the use of an oil-lubricated pistoncompressor for obtaining pure “food-grade” CO₂ is out of the question.Said piston compressor is furthermore sensitive to the presence of waterin the gaseous product to be compressed, because the water might causeirreparable damage to the moving parts of the piston compressor at thepressures that prevail therein. In addition to that, a piston compressoris characterized by a large number of moving parts, which requiremaintenance, whilst furthermore its efficiency is relatively low.

[0029] It has turned out that it is possible to obtain a much betterperformance by using a so-called screw compressor instead of a pistoncompressor. When a screw compressor is used as the compression means inaccordance with the invention, not only a more compact installation isobtained, but above all a screw compressor which distinguishes itselffrom the known piston compressors by a higher efficiency and a smallernumber of moving parts. In particular the latter fact means asignificant simplification as regards maintenance, whilst also the riskof malfunctions is reduced.

[0030] Since a screw compressor is smaller than the usual pistoncompressor, both as regards its dimensions and as regards its weight, itis possible to exchange the screw compressor for another screwcompressor in its entirety in the case of maintenance, so that anymaintenance that may be required need not be done on-site. The durationof a possible shutdown of the CO₂ recovery plant is thus reduced to ashort period, and the quick exchange of the screw compressor enables aquick restart of the apparatus. Although it is possible in accordancewith the invention to use a non-lubricated screw compressor, in whichthe screw rotors rotate freely in the pump housing, in an apparatusaccording to the invention, this has the drawback that cooling cannottake place and that consequently the process temperature of the CO₂ flowmay increase considerably. In addition to that, only small compressionratios are possible with a non-lubricated screw compressor. In order tocompress the CO₂ to the desired process pressure of 18 bar, cooling ofthe screw compressor is necessary, and it has turned out that theinjection of water into the CO₂ flow can provide the desired coolingeffect. Because the water content of the CO₂ flow increases considerablyas a result of the injection of water on the inlet side of the screwcompressor, it is necessary to separate the water from the CO₂ flowafter the compression stage.

[0031]FIG. 2 schematically shows the compression part of the CO₂recovery plant according to the invention, in which water separatingmeans 18 are arranged after the screw compressor 13. Preferably, saidseparating means 18 are in the form of a cyclone. The separated watercan be reused very effectively by returning it to the inlet side 13′ ofthe screw compressor 13. The advantage of returning the separated wateron the inlet side of the screw compressor, via the pipe 18 c, ismoreover that any water-soluble components (such as alcohols etc) thatmay be present in the CO₂ flow will be dissolved in the injected water.This enables a much smaller construction of the means for removingwater-insoluble impurities from the product, in this case the gaswashing installation which is indicated at 12 in FIG. 1, which in turnmeans lower installation costs.

[0032] On the other hand, the gas washing installation 12 can be leftout altogether in that the gas washing installation 12 can be fullyincorporated in the water separating and returning means 18-18 c of thescrew compressor 13. Furthermore it has turned out that a screwcompressor is not adversely affected by the compressing and pumping ofwater-containing CO₂. This in contrast to the piston compressor that hasbeen used so far, which is only capable of compressing “dry” CO₂, sincethe piston compressor will otherwise be confronted with a so-calledwater hammer due to the possible presence of condensed water, which maycause considerable damage to the moving parts. In order to compensatefor any temperature increase of the CO₂ flow after the first compressionstroke, a cooler 19 may be fitted in the outlet pipe 13 a.

[0033] Analogously to FIG. 1, the compression of the CO₂ flow by meansof the screw compressor is carried out in two stages, in which twoseries-arranged screw compressors are used in the process line. In thecase of water-injected screw compressors, each compression stage willconsist of a screw compressor 13 as well as water separating andreturning means 18-18 c, for example in the form of a cyclone. The screwcompressor of the second compression stage is fed with the compressedCO₂ flow, which leaves the water separating means 18 via the pipe 18 a.The dissolution of water-insoluble components can be enhanced byreturning the separated water to the inlet side of the screw compressorin question in each compression stage and thus injecting it again, sothat the gas washing installation 12 (see FIG. 1) can be left outaltogether. The two compression stages, each comprising a waterseparating device 18, also function as a gas washing installation inthat case.

[0034] The screw compressor as shown in FIG. 3, which forms one of thetwo compression stages of the compression means of the apparatusaccording to the invention, has a housing 20 fitted with two screwrotors 21 and 22. The two screw rotors 21 and 22 are mounted on rotorshafts 21 a and 22a respectively, which are mounted in the housing 20 bymeans of suitable bearings 23 (ball bearings, for example). The rotorshaft 21 a is connected to driving means (not shown) in that case, sothat both screw rotors 21 and 22 can be rotatably driven via the shaft21 a. The helical groove 24 of the screw rotor 21, together with thecorresponding groove 25 of the other screw rotor 22, forms thecompression chamber, as is clearly shown also in FIGS. 4A-4C. Thecompression chamber, which is indicated by the hatched portion 30 inFIGS. 4A-4C, has a variable volume V, which will initially increase tothe situation that is shown in FIG. 4A as a result of the rotation ofthe two screw rotors 21 and 22. The chamber volume V formed by thecompression chamber 30 then increases from 0 to a maximum value in thatcase, see FIG. 4A, in which situation the compression chamber 30 is incommunication with the inlet port 26 (see FIG. 3) in the wall of thecompressor housing 20. The CO₂ flows into the compression chamber 30 ata nearly constant pressure in that situation.

[0035] As a result of the shape of the compressor housing 20, thecompression chamber 30 will be shut off upon reaching its maximum volumeV during the compression phase. The rotation of the screw rotors 21 and22 via the rotor shaft 21 a has progressed so far by now that thecompression chamber 30 that has been formed is no longer incommunication with the inlet side 26. Further rotation of the screwrotors 21 and 22 will cause the compression volume V of the compressionchamber 30 to decrease (see FIG. 4B). The compression causes the outletopening at the rear side of the screw compressor to be opened as aresult of the presence of an overpressure. The outlet opening on thehigh-pressure side of the screw compressor is indicated at 27 in FIG. 3.

[0036] The reduction in volume from a maximum volume of the compressionchamber 30 just before the inlet opening 26 closes to the volume at theend of the compression stroke just before the outlet opening 27 opens iscalled the compression ratio of the screw compressor. When the outletport 27 opens, the compressed CO₂ is displaced to the pressure pipe,indicated at 13 a in FIGS. 1 and 2 and at 13 b in the second compressionstage, whilst the pressure remains practically the same.

[0037] When a suitable screw compressor is used, the volume V of thecompression chamber 30 has been reduced to ⅕ of its volume at the end ofthe compression stroke. Because several grooves of the screw rotors 21and 22 mate in succession, thus forming successive compression chambers30 (at various stages of the compression cycle), the compressed gas flowis hardly interrupted.

[0038] In order to obtain the desired process pressure of 18 bar, thetwo screw compressors must be geared to each other as regards the volumeratio and the volume flow. This is achieved in part by driving the twoscrew compressors with a suitable number of revolutions. Preferably, thescrew compressors are driven simultaneously by driving means so as toobtain compression values which are precisely geared to each other. Thecompressors may be driven via a belt transmission, for example, whereinthe number of revolutions for each screw compressor can be set toachieve the most effective operational conditions by selecting adifferent transmission ratio.

[0039] Although the CO₂ recovery plant according to the invention asdescribed above comprises two compression stages, it is also possible touse only one screw compressor having the compression ratio that isrequired to achieve the final pressure of the gaseous product, forexample a compression ratio of 1:18 or 1:20.

[0040] Furthermore it should be noted that the CO₂ recovery plantaccording to the invention has been described above as an apparatus forrecovering CO₂ from a CO₂-containing gaseous product which has beenreleased in the course of a beer brewing process. The apparatusaccording to the invention can also be used in other processes in whichcarbon dioxide is released, however, for example in the fermentation ofwine or other beverages obtained on the basis of fermentation processes,as well as in the industrial recovery of CO₂ from process gas and therecovery of CO₂ from natural sources, such as air.

1. An apparatus for preparing pure carbon dioxide (CO₂) from a gaseousCO₂ containing product, which apparatus comprises a processing linewhich comprises: an inlet for the gaseous product; means for removingwater-soluble impurities from the product; compression means forcompressing and pressurising the product[[,]]; cleaning means forremoving further impurities from the product, and storage means forcollecting and storing the purified CO₂, wherein said elements areinterconnected by means of suitable piping, and wherein said compressionmeans comprise at least one screw compressor.
 2. The apparatus accordingto claim 1, wherein the screw compressor is a water-cooled screwcompressor.
 3. The apparatus according to claim 2, wherein separatingmeans are arranged directly after the screw compressor for separatingwater from the compressed product.
 4. The apparatus according to claim3, a water outlet of the separating means is connected to a water inletof the water-cooled screw compressor.
 5. The apparatus according toclaim 3, wherein the means for removing water-insoluble impurities fromthe product form part of the water separating means of the screwcompressor.
 6. The apparatus according to claim 3, wherein theseparating means comprise a cyclone.
 7. The apparatus according to claim1, wherein the compression means comprise two screw compressors whichare arranged in series.
 8. The apparatus according to claim 7, whereinone driving means is used for driving the two screw compressors.
 9. Theapparatus according to claim 8, wherein the driving means drive thescrew compressors by means of one belt transmission.
 10. The apparatusaccording to claim 8, wherein the driving means drive the screwcompressors by means of separate belt transmissions.
 11. An apparatusfor preparing pure carbon dioxide (CO₂) from a gaseous CO₂ containingproduct, said apparatus comprises a processing line comprising: an inletfor the gaseous product; a gas washing/rinsing device for removingwater-soluble impurities from the product; a compressor unit forcompressing and pressuring the product; a separating unit for removingfurther impurities from the product; and a storage unit for collectingand storing the purified CO₂, wherein said inlet, gas washing/rinsingdevice, compressor unit, separating unit, and storage unit areinterconnected by suitable piping, and wherein said compressor unitcomprises at least one screw compressor.